DE 10 2011 005 842 A1 discloses an automatic parking brake for use in a vehicle, said parking brake can exert a clamping force on a brake disk so as to secure a stationary vehicle. The clamping force is generated by means of a spindle nut and a brake piston that are displaced by means of a brake motor and grip the brake disk between two brake pads during an application of said parking brake. In addition, the brake piston can be influenced by means of a hydraulic brake fluid which is achieved by means of the regular vehicle brake (service brake). As a result of braking procedures that are performed whilst travelling it is possible for the brake disk to become warm or rather hot. As a consequence, during the procedure of securing the stationary vehicle by means of the parking brake, the problem arises that the increased temperature changes the coefficient of friction and the expansion of the brake disk.
If the brake disk cools down after an application of the automatic parking brake, a loss of clamping force can result. Generally, a further application of the automatic parking brake is therefore initiated sometime after the initial application and said further application of the automatic parking brake is calculated and performed on the basis of the brake disk temperature. If the determined brake disk temperature exceeds a determined value, it is assumed that the loss of clamping force experienced by the automatic parking brake is not negligible if the brake disk has cooled down after some time. Consequently, a further application of the parking brake is performed that is intended to compensate for the loss of clamping force. If on the other hand the brake disk temperature is less than the predetermined temperature value, then a further application of the automatic parking brake is not performed because the function of the automatic parking brake is not expected to be impaired.
However, the above described, exclusively “temperature-based” parking brake system has the disadvantage that, despite the further application of the automatic parking brake, a part of the clamping force can be lost if a thermal relaxation occurs. This is in particular the case if long braking maneuvers or multiple intense braking maneuvers are performed immediately prior to the application of the parking brake. The braking maneuvers cause in particular the brake disk and the brake pads to heat up and expand. During the subsequent cooling process and in the case of simultaneously locked parking brakes, the components “shrink” causing a part of the clamping force to be lost. The brake disk temperature model (BTM) provides in this case merely a starting point with respect to the temperature of the brake disks. The BTM is not precise particularly as a result of the changed flow behavior (e.g. by means of a spoiler or hub caps), the material (steel or aluminum rims) and also the environmental conditions. Depending upon the configuration, “estimation errors” of 50° C. to 100° C. are possible.
Conventional brake systems frequently further apply the automatic parking brake without taking into account how much it is necessary to further apply the automatic parking brake or rather when it is the optimum time for further applying said parking brake. Under certain circumstances, the automatic parking brake is even further applied on a number of occasions which necessarily leads to the open-loop control device running on for a long period of time and consequently increases the loading on the components since the electric brake motor together with the gear mechanism must run on more than one occasion against a pre-stressed system.